1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image. In addition, the present invention also relates to a method for preparing the toner, and an image forming method and an image forming apparatus using the toner.
2. Discussion of the Background
Recently, a need exists for an electrophotographic image forming apparatus which can produce high quality images. In attempting to fulfill the need, various electrophotographic image forming apparatuses and toners have been proposed and developed. In order to produce high quality images by a toner, the toner preferably has a sharp particle diameter distribution. Specifically, when a toner has a sharp particle diameter distribution, the toner particles can exhibit almost the same behavior in an image developing process, and thereby images with improved fine dot reproducibility can be produced.
However, conventional toners having a relatively small particle diameter and a relatively sharp particle diameter distribution tend to cause a cleaning problem in that toner particles remaining on a surface of an image bearing member (such as photoreceptors and intermediate transfer belts) cannot be well removed with a cleaning blade, resulting in formation of images with background development. In attempting to solve the cleaning problem by toner, various proposals have been made. For example, a toner whose particle form is changed from the spherical form to irregular forms (this particle form change is hereinafter referred to as deformation) is proposed. By deforming a toner, the fluidity of the toner deteriorates and thereby toner particles remaining on an image bearing member can be blocked with a cleaning blade. Therefore, the residual toner particles can be well removed with the blade. However, when deformation of a toner is excessively performed, the behavior of the toner particles thereof becomes unstable, and thereby the fine dot reproducibility of the toner is deteriorated.
In addition, by deforming a toner, the fixability of the toner tends to deteriorate although the cleanability thereof is improved. Specifically, a toner layer constituting a toner image has low density (i.e., there are many voids in the toner layer), and therefore the toner layer has low heat conductivity, resulting in deterioration of the low temperature fixability of the toner. This phenomenon is remarkable when the fixing pressure is relatively low.
Published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 11-133665 discloses a toner constituted of a polyester resin and having a Wadell working sphericity of from 0.90 to 1.00. This toner has substantially spherical form, and therefore the toner has poor cleanability.
When toner particles are prepared by polymerization methods, suspension polymerization methods, emulsion polymerization methods and solution suspension methods can be typically used. Among these methods, the emulsion polymerization methods and solution suspension methods can easily produce deformed toner particles. However, emulsion polymerization methods have a drawback in that residual monomers (such as styrene monomer), emulsifiers and dispersants cannot be perfectly removed from the reaction product. Such toner pollutes the environmental.
When toner particles having projected portions and recessed portions are mixed with an external additive (i.e., fluidizer) such as silica, particles of the external additive adhered to projected portions tend to move to recessed portions after long repeated used because adhesiveness of the external additive to recessed portions is relatively weak compared to adhesiveness thereof to recessed portions. In this case, the toner contaminates the image bearing members such as photoreceptors and fixing members such as fixing rollers, resulting in deterioration of image qualities and occurrence of jamming in a fixing device.
The solution suspension methods, in which a toner composition liquid, in which toner constituents are dissolved or dispersed in an organic solvent, is granulized in an aqueous medium to prepare toner particles, have an advantage in that polyester resins, which have relatively good low temperature fixability, can be used as binder resins. In the solution suspension methods, a high molecular weight component is included in a toner composition liquid and therefore the toner composition liquid tends to have a high viscosity. Therefore, the solution suspension methods tend to have a production problem in that toner particles cannot be easily prepared. JP-A 09-15903 discloses a toner, which has spherical form and whose surface is roughened to have asperities, in attempting to impart good cleanability to the toner. However, the asperities of the surface of the toner do not have regularity, and therefore the toner has poor charge stability. In addition, controlling of molecular weight of the binder resin thereof is not performed, and therefore a good combination of durability and releasability cannot be imparted to the toner.
Toner can also be prepared by pulverization methods. Pulverization methods typically includes kneading toner constituents such as binder resins, colorants and additives (such as charge controlling agents) upon application of heat thereto, pulverizing the kneaded mixture, and then classifying the pulverized mixture to prepare toner particles. The pulverization methods have the following drawbacks.    (1) toner having a small average particle diameter cannot be provided (i.e., the particle diameter has a certain lower limit);    (2) it is impossible to properly position toner constituents in toner particles (for example, to position a charge controlling agent in a surface portion); and    (3) when the added amount of a charge controlling agent is increased, a filming problem in that a film of the charge controlling agent is formed on the surface of carrier particles used for the developer and/or image bearing members, resulting in deterioration of image qualities and a fixing problem in that toner images cannot be firmly fixed to receiving materials particularly at a low fixing temperature are caused.
PCT patent application publications Nos. 2003-515795 (WO01/040878), 2006-500605 (WO2004/019138) and 2006-503313 (WO2004/019137), and JP-A 2003-202708 have disclosed to use layered inorganic materials, in which interlayer ions (such as metal cations) are modified with an organic cation, as charge controlling agents of toners. However, the toners including such layered inorganic materials also have the drawbacks mentioned above.
Because of these reasons, a need exists for a toner which has good chargeability and good releasability so as to be used for oil-less fixing devices and which can produce high quality and high definition images without causing the cleaning problem, filming problem and fixing problem.